There had to be a balance between the “right to know” and “the right to privacy”, he said.

All messages sent on WhatsApp have end-to-end encryption, meaning messages are unreadable if they are intercepted by anyone, including law enforcement and WhatsApp itself.

Masood was born Adrian Elms in Dartford, Kent

So while Masood’s phone is believed to have connected with the app, police may not know what, if anything, was communicated.

The Facebook-owned company, which has a billion users worldwide, has said protecting private communication was one of its “core beliefs”.

Tim Cook, the chief executive of Apple which also uses end-to-end encryption, has previously said it would be “wrong” for governments to force Apple to “build a back door” into products.

But Ms Rudd said: “I would ask Tim Cook to think again about other ways of helping us work out how we can get into the situations like WhatsApp on the Apple phone.”

PA/FacebookImage caption PC Keith Palmer, Kurt Cochran and Aysha Frade all died in the attack (left to right)

The victims of the Westminster attack were commemorated at the beginning of England’s World Cup qualifier against Lithuania at Wembley Stadium.

Before kick-off, four wreaths were laid in the centre of the pitch by Metropolitan Police Acting Commissioner Craig Mackey, London Mayor Sadiq Khan, FA chairman Greg Clarke and Culture Secretary Karen Bradley.

A minute’s silence was also observed by fans and players.

Four wreaths for the four victims of the Westminster attack were laid in the centre of Wembley Stadium’s pitch before England’s World Cup qualifier with Lithuania

Europol director Rob Wainwright echoed Ms Rudd’s call for changes.

“I would agree something has to be done to make sure that we can apply a more consistent form of interception of communication in all parts of the way in which terrorists invade our lives,” he told Andrew Neil on Sunday Politics.

Masood, 52, killed three people and injured 50 when he drove a car into pedestrians on Westminster Bridge on Wednesday.

He then fatally stabbed a police officer before being shot dead by police – all within 82 seconds.

Ms Rudd would not confirm who shot Masood, amid claims it was a bodyguard for Defence Secretary Sir Michael Fallon.

Metropolitan Police Acting Commissioner Craig Mackey has laid a wreath in commemoration of the Westminster attack victims at Wembley Stadium, before England’s World Cup qualifier with Lithuania.

He said: “I have been humbled by the support and gratitude from the public towards our officers the last few days.”

On Saturday the Metropolitan Police said they believed Masood acted alone.

But they added they were also “determined” to find out whether he had been inspired by terrorist propaganda.

Scotland Yard said it was possible “we will never understand why he did this”.

On Sunday the police investigation into the attack continued. In Birmingham, counter-terrorism officers were seen searching a house where friends of Masood lived.

A 58-year-old man, who was arrested in Birmingham the morning after the attack under the Terrorism Act, remains in custody, and a 32-year-old woman who was arrested in Manchester, remains on police bail, Metropolitan Police have said.

Eleven people were initially arrested over the incident and nine people in total have been released without charge.